1. Field of the Invention
The present invention relates to a motor control apparatus and, more particularly, to an apparatus for controlling a motor so as to smoothly rotate at a stable rotational frequency. More specifically, the present invention relates to an improvement of a control apparatus for a cylinder motor which is used in a video tape recorder and for a spindle motor which is used in an OA (office automation) machine such as a flexible disk drive, hard disk drive or an optical disk drive.
2. Description of the Related Art
FIG. 3 shows a three-phase motor control apparatus as an example of a conventional motor control apparatus.
The motor control apparatus is composed of a motor 5, a sensing circuit mainly constituted by Hall elements 1, ..., an input amplifier 2, a driving signal generator 3, and an output amplifier 4 which are connected with each other in that order so as to make a loop.
The motor 5 is a three-phase star connection type and has three coils 5a, 5b and 5c. A driving supply voltage Vcc' is applied to the common connection point of these three coils.
The input amplifier 2 amplifies minute signals. It is mainly composed of three differential amplifiers 2a, ....
The output amplifier 4 is mainly composed of power amplifiers 4a, 4b and 4c and controls the driving current which flows from the connection to which the supply voltage Vcc' is applied to the motor 5 through the coils 5a, 5b and 5c.
The operation of the motor control apparatus having the above-described structure will now be explained. The outputs of the Hall elements 1, ... for detecting the rotating state of the motor 5 are first supplied to the input amplifier 2. In the input amplifier 2, the differential amplifiers 2a, ... amplify and shape the wave forms of the outputs of the Hall elements 1, ... and supply detection signals S to the driving signal generator 3.
The driving signal generator 3 synthesizes the three detection signals S at a predetermined ratio and, if necessary, delays them, thereby generating three driving signals D having a predetermined wave form and different phases and outputting them to the output amplifier 4.
In the output amplifier 4, the power amplifiers 4a, 4b and 4c amplify the respective powers of the driving signals D, and generate the driving currents in accordance with the wave form. The thus-generated driving currents flow in the coils 5a, 5b and 5c, thereby driving the motor 5.
With the rotation of the motor 5, the detection signals S are continuously generated, so that the operation repeats from the first step. In other words, the signals containing control information and the driving power information are sent along the feedback loop in the form of different signals, whereby the rotation of the motor 5 is maintained.
The rotational structure of the motor 5 will now be explained in more detail. The motor 5 has a magnetized rotor. The rotor is rotated by the rotary magnetic field on the stator side which is generated by the driving currents applied to the coils 5a, 5b and 5c. The rotation of the rotor rotates the motor 5. The rotating state of the motor 5 is determined by the number of stators and rotors, the positional relationship between the stators and the rotors, the wave form of the driving currents, etc. Therefore, it is desirable to make the wave form of each driving current as close to that of a sine wave as possible in order to obtain a smooth and stable rotation.
In a conventional motor control apparatus, however, the driving signal D is generated by amplifying the detection signal S and shaping the wave form thereof. As a result, the wave form of the original detection signal S influences the wave form of the driving signal D. In addition, since the wave form of the detection signal S depends on the positional relationship between the detecting element such as the Hall element and the element which is attached to the motor so as to be detected by the detecting element, and the sensitivity of the detecting element, the detection signal S actually has a wave form close to a trapezoidal or pulse shape instead of that of a sine wave. Such a non-ideal wave form of the driving signal D causes a fluctuation in the torque, thereby making it impossible to control the motor so as to have a smooth and stable rotation.